Synthesis of Poly(<i>n</i>-butyl acrylate)-<i>block</i>-poly(acrylic acid) Diblock\nCopolymers by ATRP and Their Micellization in Water

Abstract

Amphiphilic diblock copolymers, poly(<i>n</i>-butyl acrylate)-<i>block</i>-poly(acrylic acid) (P<i>n</i>BA-PAA), with\nnarrow molecular weight distribution (PDI ≤ 1.07) were prepared by atom transfer radical polymerization (ATRP)\nof <i>n</i>-butyl acrylate and <i>tert</i>-butyl acrylate (<i>t</i>BA), followed by selective acidolysis of the P<i>t</i>BA block. These polymers\npossess a soft P<i>n</i>BA hydrophobic block with a constant chain length of 90−100 monomer units and pH- and\nionic strength-sensitive hydrophilic PAA block with DP_PAA = 33−300 AA monomer units. They were expected\nto form stimuli-responsive micelles. The block copolymers with DP_PAA ≥ 100 are directly soluble in water at pH\n> 4.7. Pyrene steady-state fluorescence spectroscopy and fluorescence correlation spectroscopy (FCS) studies\nindicate the existence of a very low critical micelle concentration (cmc ∼ 10^-8 mol/L). The number-average\nhydrodynamic radii of the micelles, as determined by FCS, range from 28 to 55 nm, depending on the PAA\nblock length. FCS data indicate that micellar sizes significantly decrease upon dilution for salt-free systems. This\nis attributed to a dynamic, but kinetically controlled, behavior of these self-assembled nanostructures. In saline\nsolutions the micellar sizes remain constant above the “apparent” cmc (cmc*), which is attributed to slower\ndynamics of unimer exchange between micelles.